The New Madrid earthquake zone: a cold, dying fault system?

Recent scientific results give new insights into the mysterious earthquakes in the central U.S. and how society should address the resulting hazard.

These earthquakes are frustrating for local officials, business owners, insurers, and the public in the New Madrid seismic zone, which extends approximately from southern Illinois to southern Tennessee and also includes parts of Arkansas, Kentucky, and Indiana. It is hard to find out how serious a hazard they pose or what it makes sense to do. Recommendations abound but asking simple questions reveals how little is actually known. This is exciting to scientists – who like to tackle the unknown – but frustrating to everyone else.

At present we don’t know why the earthquakes occur, when and why they started, if and when future large earthquakes will occur, how serious the danger of such earthquakes is, and how society should confront it. Moreover, because large earthquakes here are far rarer than in many other seismic zones, these issues are hotly debated but will not be resolved for hundreds or thousands of years.

As a result, communities in the New Madrid seismic zone have a tough problem. They have to decide whether to follow FEMA’s advice to adopt building standards for earthquake resistance as stringent as those in California, or to adopt more modest standards and use the billions of dollars saved otherwise. The stakes are high. For example, bringing one hospital in Memphis to California standards cost $100 million.

The major hazard could come from large (magnitude 7) earthquakes like those that shook the area in 1811 and 1812. Houses collapsed in the tiny Mississippi river town of New Madrid, Mo., and minor damage occurred in St. Louis, Louisville and Nashville. The smaller earthquakes that continue today are typically more of a nuisance than a catastrophe. The largest in the past century, the 1968 southern Illinois earthquake (magnitude 5.5), was widely felt and caused some damage but no fatalities. However, if large earthquakes like those of 1811-12 occurred again, they would be very destructive. Hence we would like to know if and when large earthquakes will occur again.

New insight is emerging by combining results from the Global Positioning System (GPS), which measures motions of the ground to incredible accuracy - fractions of an inch per year - and measurements of the temperatures of rock deep below the New Madrid zone. Together, these shed light on why past large earthquakes happened and when they may happen again.

One important question is why large earthquakes occur at New Madrid rather than on similar geologic structures elsewhere in the mid-continent. Because hot rocks are weak, it had been suggested that large earthquakes occur because New Madrid rocks are hotter. However measurements in boreholes of heat coming to the Earth’s surface show that New Madrid rocks are no hotter and therefore weaker than in other parts of the central U.S.

This result seems to settle a crucial aspect of assessing the earthquake hazard - whether New Madrid is special or simply where large. earthquakes have occurred in the past few thousand years. The heat flow, like other geological data, show no obvious difference between New Madrid and similar places that haven’t had large earthquakes during the same time.

The new heat flow results fit into a growing idea that earthquakes migrate among similar faults, such as the Meers fault in Oklahoma that appears to have been active about 10,000 years ago but show no activity today. Geologic studies find that New Madrid earthquakes comparable to those of 1811-1812 occurred about 1450 and 900 AD. However, because this fault system has not generated significant topography, it is likely to have “turned on” relatively recently, perhaps within the past few thousand years.

Thus it looks like prior earthquakes were concentrated on other faults, and future earthquakes will occur somewhere else when New Madrid “shuts down.” Once this happens, it may be a very long time -- thousands of years or longer -- before New Madrid becomes active again. Migrating earthquakes also occur in the interior of other continents, like Australia. This is very different from the way earthquakes occur on boundaries between plates, like the San Andreas fault along the boundary between the Pacific and North American plates. Because the plates keep moving, earthquakes continue to occur in the same places.

Although we don’t know when the New Madrid fault system will shut down, it may be dying today. The recent cluster of earthquakes may be coming to an end. A strong case for this possibility comes from Global Positioning System measurements showing that motion across the New Madrid zone currently is either very slow or zero. Because this motion has to accumulate for many years to cause a large earthquake, it will be at least hundreds of years, and perhaps much longer, before another large earthquake happens. In this idea, the small earthquakes that continue today are aftershocks of the 1811-1812 earthquakes.

The new results are exciting because until recently about all we could say was that future earthquakes might occur in places where past ones had. Now we can actually test that idea by looking at motion accumulating for possible future earthquakes. Although we can’t be sure yet, the longer GPS data continue to show essentially no motion, the more likely it seems that the fault is shutting down and won’t cause large earthquakes for a very long time. It’s time to start thinking about this possibility and to use what we’re learning to improve estimates of the hazard from future earthquakes.

In particular, it looks like rushing to adopt California-style building codes would be a mistake. The federal government assumes that the earthquake hazard in the central U.S. is the same as in California, based on very uncertain estimates of the maximum shaking expected at a point on the ground once in 2,500 years. A more useful estimate is to consider the likely damage to a building during its actual life, which is about 50 years. Estimated this way, the earthquake risk in the New Madrid zone is about one-tenth to one-third that of California's. Hence it would make more sense to adopt less stringent measures, and use the additional resources otherwise. After all, money spent strengthening schools isn't available for teachers' salaries, upgrading hospitals may mean treating fewer uninsured patients, and stronger bridges may result in hiring fewer police officers. Although the proposed code may over time save a few lives per year, the same money invested in other health or safety measures could save many more. Thus although research will continue, we can use its results already to make better policy choices. Picture caption: The $100 million spent bringing the Memphis Veterans’ Administration hospital to California earthquake resistance standards would have been better spent by using a less stringent standard, releasing funds for patient care.

Seth Stein William Deering professor Department of Earth and Planetary Sciences Northwestern University